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Small fiber nerves vs large fiber nerves

  Can you please clarify differences between the peripheral nerves:
    - there are small and large sensory fibers that carry different info?
    - do these fibers simply differ in diameter, or are they made from
        different materials (and so could be affected by different compounds)?
    - these sensory fibers differ in construction from large fibers that
        carry muscle signals (and so again as a class could be affected
        differently by the same compound)?
    - nerve conduction tests primarily measure large diameter fibers
        (sensory and motor)?
    - is taste (tongue sensory signals) carred by large or small sensory fibers?
  Thanks in advance
Dear Paul,
I assume that your question concerns with sensory nerve fibers rather than motor nerve, that is also a part of the peripheral nervous system.
You are correct that there are in general two types of sensory nerve fibers,
the small and the large. The small are composed of two types, the C fiber that is thin (diameter 0.4-1 micrometer) and unmyelinated (no insulator or myelin) hence also called the slowly conducting fiber. The second type is the more rapid, thinly myelinated A-delta fiber (1-5 micrometer in diameter). Both carry types carry pain sensation and others (intense pressure, cold, and others depending on the receptor type). The large fibers carry touch and pressure as well as joint position sense. These fibers are myelinated, hence faster in conducting impulses from the periphery to the brain.
When you touch a hot plate for example, you will feel pain/hot right away, that is the A-delta fibers, and then later (seconds or minute later) you feel deep achy burning pain, that is the slow C fiber...
You are also correct that the EMG/NCS measures only the faster and hence the larger fibers and does not exclude the possibility of small fiber neuorpathy if it is normal.
Good luck and I hope the above information helps.

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